Eclipsing binaries vs transits

Observing at Keck, Christmas Eve 2010.

It’s Christmas Eve and I’m starting a five-night observing run at the Keck Telescope using a high-resolution spectrograph (HIRES) to search for exoplanets. In the photo here, I am communicating with the telescope operator, Terry, by polycom. He is on the summit of Mauna Kea at 14,000 ft where the air is thin and I see that he has oxygen flowing. I’m glad that I’m working in comfort at Keck HQ in Waimea.

Tonight, I’m using the Doppler technique to measure the velocities of stars.  Orbiting planets tug their host stars around a common center of mass.  This reflex stellar velocity is largest for massive planets.

When small stars eclipse larger stars, the brightness dip can be virtually the same as those for transiting gas giant planets. To confirm a transit candidate as a planet, Doppler measurements are needed to determine the mass of the transiting object. The Kepler team has a massive follow-up campaign (led by Dr. Geoff Marcy at UC Berkeley) using the same setup that I’m using now.  Dr. Natalie Batalha (Deputy Scientist for the Kepler project) explains that the team is also eager to have others helping and to have Planet Hunters combing through the data. Watch for a blog post by Dr. Batalha here soon!

Some of you have asked how many consecutive low points you should see during a transit. That depends on how close the planet is to the star. Close planets orbit faster and transit in a few hours while more distant planets take several hours to transit. You should look for more than one low point.  Since the brightness measurements are taken every 30 minutes, a 3 hour transit would consist of just 6 low points. However the ingress, or first transit point, might be transitional and not reach the transit floor. Ditto for the egress, or last transit point).

The light curves for eclipsing binary stars are quite spectacular – they remind me of sketches I used to make with a “spirograph” toy I had as a kid. Some of the planet hunters have called this a shutter effect and I’ve written a quick program to demonstrate what is happening. In the Figure below, I created a theoretical light curve for a contact eclipsing binary with an orbital period of just 6 hours. If we had observations of this star every few minutes, then the light curve would look similar to a sine wave (left plot). However, if we observe this star less frequently (a slow “shutter speed”), then some interesting patterns emerge. The plot on the right in the Figure below shows an under-sampling of the light curve over 30 days. The pattern is similar to what appears in some of the eclipsing binary curves you are finding in the Kepler data.



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23 responses to “Eclipsing binaries vs transits”

  1. Brian Proskin says :

    I’m new to Planet Hunters and would Appreciate 20 or more examples of Biaries found and Exoplanets found so we can get a feel for what an actual discovery would look like

    • debrafischer says :

      Hi Brian,
      We set up some “fake” transits to be mixed in with the regular data, but there seems to be a bug – these plots aren’t coming up. I’ll check on this again today. The great thing about the fake data sets are that we use real light curves, but just inject theoretical (with typical noise) transit dips. Once you classify a star, if there was a fake transit in the light curve, you will be notified and those transit dips should be highlighted. This teaches our users, and it also provides a check on the statistics of this technique. It might be another day or two before this is implemented (b/c of the holiday). Best, Debra Fischer

  2. Andres Eloy says :

    Ok Dra. Fisher, now I understand, the mechanism that produces such strange shapes. Will continue in the hunt for exoplanets.

  3. tjaracas says :

    thanks for this good explanation about EB on xmas eve. can you please show in your next blog a real life example of a possible earth sized planet transit from the available kepler data. I would just love to see a 3 to 6 point transit to be able to interpret it correctly. I fully understand that all transits can look different due to the 30 minutes brightness measurements distance to the star type of star etc. .

    • debrafischer says :

      Hi tjaracas, Check the blog by Matt Giguere . Also we are supposed to be including “synthetic” or fake transit events to help you learn – just seems to be a bug in the code that serves those up to the participants. We’ll try to get this fixed in the next day or two!

  4. Tjapko Smits says :

    Hello Deborah, Thanks for answering me. The problem I do have with Matt’s explanation is the vertical brightness scale in his example. In his example he did put 3 types of planet transits together in 1 graphic. The earth like planet transit is hardly noticeable because it normally should have an adjusted vertical scale. Please correct me if I am wrong. I also went to the Kepler site directly where you can download the data for the 8 found exo-planets. Very helpful data also. I understand that for earth sized planets we need more time as explained in the kepler site but still I would like to see an example from real “public” data for a possible earth sized candidates. Just to satisfy my curiosity and analyse the graphics as good as possible.

  5. Derek N says :

    @ Brian –

    Here is a collection that shows examples of what you should be looking for:

  6. Tim Wicks says :

    Do you have a presence on facebook? I can’t seem to find Eclipsing binaries vs transits « Planet Hunters Blog on there and I would like to connect with you there. I like your writing style, thanks Tim Wicks

  7. Tjapko Smits says :

    Just like to answer my own question related to earth sized planets transits. In a new blog from debra fisher about transits it became clear to me that so far most earth sized planets are found together with “jupiter” sized planets. In all the examples the lightcurve processed by the jupiter sized planet will cause the y-axis to decrease and be minimal for the detection of planets with earth size radius and thus making this work even more interesting. So at the end maybe the example from Matt was just misinterpreted by me and correct after all.

  8. mutabilitie says :

    @Tjapko Smits
    ‘So far most earth sized planets are found together with “jupiter” sized planets.’

    That’s kind of obvious, though, isn’t it?
    Jupiter-sized planets are easiest to spot, so stars which have them are most likely to catch people’s attention and be given a closer look, and if there are any earth-sized planets lurking around in the vicinity, chances are that they’ll then be spotted as well…

  9. mutabilitie says :

    Bleh, I just read that again and realised how patronising I sound. That wasn’t my intention, sorry.

  10. Tjapko Smits says :

    @mutabilitie. No problem. IMHO we still don’t know yet if multi planet solar systems are that obvious out there. The latest transit blog made me aware of my misinterpretation of a previous blog from Matt Giguere who explained the tiny variation to look for when spotting earth sized transit together with a jupiter sized planet.

  11. mteleshoff says :


    Sorry, the link you posted doesn’t work. I think it’s missing one character from the end of the URL, as collections seem to have URLs in this form:

    where n is a numeral and x is a letter.

  12. mteleshoff says :


    Sorry, although I still can’t access the collection you linked above, what I said about the collection URL’s isn’t true; they vary in form.

  13. Derek N says :

    Hi mteleshoff,

    I deleted that collection, try this one instead:

  14. M2 says :

    That is precisely the effect of aliasing!!

  15. Prof.Leo says :

    Como posso saber se estou indicando corretamente os trânsitos ? Estou seguindo as instruções mas é melhor consultar para evitar êrros na observação.
    Prof. Leo

    • Barbara says :

      Prof Leo, acho que não tem como saber se estamos certos. Pelo que entendi, a equipe vai verificar os trânsitos mais identificados pelos voluntários para mais tarde determinar o que são (planetas, binárias eclipsantes, etc).

  16. Bogdan says :

    I think the planethunters website doesn’t have enough tools for data analysis. You cannot do anything with the graph. For example the axis scaling function and possibility to merge quickly more quarters would be very usefull

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